1. Field of the Invention
This invention is directed to the fabrication of compound semiconductor films in general. More particularly, it relates to the formation of CdS compound semiconductor films.
2. Prior Art
Compound semiconductor films are employed in the design and fabrication of photoconductors, light activated display devices, solar cells and thin film transistors. Typical materials found to be useful in such applications are formed from Group II-VI and Group III-V compounds.
It is recognized that considerable advantages, with respect to photoelectric characteristics, are obtained by the fabrication of compound semiconductor films having larger crystallite textures. Such crystalline structures on amorphous substrates are usually obtained by a recrystallization process.
Prior art recrystallization processes may be characterized as involving Microzone recrystallization from the melt (1.) and recrystallization with a metal flux (2.). The former process (1.) is exemplified by A. R. Billing, J. Vac. Sci. Technol. 6, 757 (1969). This process has been employed with InSb and is considered limited to lower melting point compounds having comparable vapor pressure components. It has the added disadvantage of requiring a cover film to preclude droplet formation. The temperature and vapor pressure considerations, critical to the melt process, limit the design flexibility of devices fabricated via this process in that many compounds are excluded. Typical excluded compounds are CdS, CdSe, GaAs, GsP, InAs, and InP.
The latter process (2.) disclosed by Gilles, Van Cakenberghe et al has been extensively studied in connection with CdS. The recrystallization techniques of Gilles and Van Cakenberghe, (J. Gilles and J. Van Cakenberghe, Nature 182, 862 (1958)) and Vecht and Apling, (A. Vecht and A. Apling, Phys. Stat. Sol. 3, 1238 (1963)) allow the formation of rather large monocrystalline regions within such evaporated films. However, these techniques suffer from two distinct disadvantages. First, the films are automatically doped strongly with Ag or Cu and, second Cu and Ag segregation on intercrystalline grain boundaries is difficult to avoid, (R. Addiss, U.S. Government Technical Rept. ASD-61-11 (1962)).
In 1972, Herinckx, et al., (C. Herinckx, W. DeSutter, A. Fourdeux, and N-Terao, Phys. Stat. Sol. (a) 10,387 (1972)) reported recrystallization of CdS films with as little as 40 ppm Ag dopant concentration. This would tend to minimize the second problem. Nevertheless, such films are still strongly doped. Recently recrystallization in CdS films has been observed without the use of Ag or Cu (W. Kahle and H. Berger, Phys. Stat. Sol. (a) 2, 717 (1970)); instead, temperatures in excess of 700.degree. C were employed to achieve the recrystallization. The recrystallization temperature of 700.degree. C for CdS films is greater than that required by the process disclosed by Applicants for the same type of initial CdS film.